Ophthalmic lenses, in particular contact lenses, which it is intended to produce economically in large numbers, are preferably produced by the so-called mold or full-mold process. In this process, the lenses are produced in their final shape between two mold halves, so that neither subsequent machining of the surfaces of the lenses nor machining of the edge is necessary. Mold processes are described, for example in WO-A-87/04390, EP-A-0367513 or in U.S. Pat. No. 5,894,002.
In the known molding processes, the geometry of the contact lens to be produced is defined by the mold cavity between the two lens forming areas of the mold halves. The edge of the contact lens is likewise formed by the mold, whereby the geometry of the edge is defined by the contour of the two mold halves in the region in which they make contact with each other or by the spatial limitation of the UV light used for cross-linking the lens forming material.
In order to produce a contact lens, usually a specific amount of a flowable lens forming material is introduced into the female mold half in a first step. The mold is then closed by putting the male mold half into place. The subsequent polymerization and/or cross-linking of the lens forming material is carried out by means of irradiation with UV light and/or by heating. In the process, either both the lens forming material in the mold cavity and the excess material in the overflow are hardened or only the lens forming material in the mold cavity is hardened, whereas the excess material in the overflow remains as “flash”. In order to obtain fault-free separation of the contact lens from the excess material, good sealing or expulsion of the excess material must be achieved in the zone in which the two mold halves make contact with each other or which defines the spatial limitation of the UV light used for cross-linking the lens forming material.
After the lens if formed, the mold is disassembled and the lens removed. Additional processing steps, such as inspection, extraction, hydration, surface treatment and sterilization may finally be performed on the lens before packaging.
The materials used for the molds in the above described processes are preferably glass or quartz.
EP-A-0985520 discloses a mold assembly for forming an ophthalmic lens, comprising a first mold and a second mold which cooperate with each other to define therebetween a mold cavity having a profile following that of the ophthalmic lens, characterized in that: at least one of the first and second molds is a coated mold consisting of a body portion including a backing surface having rigidity and mechanical strength enough to withstand a molding operation for forming the ophthalmic lens in the mold cavity, and a thermoplastic film retained on the body portion such that the thermoplastic film covers the backing surface of the body portion, the thermoplastic film having a molding surface which partially defines the mold cavity and which gives a corresponding one of opposite surfaces of the ophthalmic lens.
DE-A-19731780 discloses a process for manufacturing sensitive components, such as lenses or micro devices, wherein in the process a film is placed onto a tool insert, the component is molded onto the film between the two mold halves, i.e. the tool insert, respectively, and the film is demolded with the component adhering to it. In a preferred embodiment, the tool insert further comprises a pinch-off rim to cut of the gate, when the mold halves are closed and the film is compressed by the two mold halves, i.e. the tool insert, respectively.
U.S. Pat. No. 4,038,014 discloses a mold for forming a lens blank with a thickness of from 0.75 to 1 mm under applied heat to meniscus shape, by applying a force created by vacuum. The vacuum preferably is applied through a central hole in the mold. Further, in between the lens blank and the shaping surface, a rubber cushion or synthetic fabric is provided, which allegedly is effecting improved accuracy in lens blank conformance to the shaping surface, improved surface smoothness and improved release upon completion of a forming cycle.
US-A-2007/0132135 discloses and apparatus and method for manufacturing optical electronic components, wherein a transparent resin, e.g. epoxy resin or silicone resin, is applied to protect an optical device as well as to act as a lens. In particular, the apparatus comprises a molding cavity and a mold release film, as well as a vacuum pump to attract the mold release film towards the cavity's entire surface. The mold release film is disclosed to have excellent heat resistance and durability to cover the mold cavity during the molding process.
U.S. Pat. No. 4,540,532 discloses a mold assembly and method, for making a molded polymeric article having a predetermined shape, comprising an envelope completely encasing a quantity of a liquid polymerizable monomer mixture, the envelope being characterized in that it is flexible at room temperature, essentially impermeable to oxygen, and chemically inert to said mixture, and a mold means, including a pair of mold halves disposed on opposing sides of the envelope, for forcing the envelope and its contents to conform to the inner surfaces of both mold halves. As FIG. 1 of U.S. Pat. No. 4,540,532 shows, a polymerizable monomer mixture 10 is sealed into an envelope 11. As shown in FIG. 2 of U.S. Pat. No. 4,540,532 said monomer mixture is polymerized within the sealed envelope while it is in a mold 20, comprising a pair of mold faces 21 and 22 exerting pressure on the envelope.
The mold assembly and method of the prior art has two major drawbacks:
(1) It is difficult to first form the envelope, then introduce the liquid polymerizable monomer mixture into the envelope outside the mold halves, sealing the envelope and only then transferring the envelope to the mold halves for making the molded polymeric article. Further, such a multi-step procedure adds complexity to an industrial scale manufacturing process, which finally makes it time consuming and costly.(2) It is difficult to control the pressure between the mold halves in such a way, that the envelope sufficiently conforms to the inner surfaces of both mold halves, and at the same time not to apply excessive pressure that ruptures the envelope. In particular the sealing of the envelope is prone to rupture under excessive pressure, leading to spill-out of the liquid polymerizable monomer mixture onto the surface of the mold halves.
Accordingly, it is an object of the present invention to provide an improved mold half and mold assembly, as well as methods allowing for an industrial scale manufacturing process of molded articles with limited complexity (i.e. less handling steps) and at low cost.
It is a further object of the present invention to provide an improved mold half and mold assembly, as well as methods effectively protecting the mold surface of one or both mold halves from contact with the material forming the polymeric article at all time during manufacturing of the molded polymeric article, in particular avoiding a complex control of mold pressure.
It is still a further object of the present invention to provide an improved mold half and mold assembly, as well as methods for reproducibly conforming a mold release sheet to the contour of the lens forming area of a mold half, for manufacturing lenses with a good optical quality and at the same time a good edge quality.
It is still a further object of the present invention to provide a process for the manufacture of a contact lens, making use of a mold half and/or a mold assembly as described above.